1. Field of the Invention
The present invention relates to the delayed generation of heat from zeolites upon contact with water. More particularly, the invention relates to coating zeolite particles such that the generation of heat from the zeolite particles is delayed when contacted with water.
2. Description of the Related Art
Zeolites are porous crystalline aluminosilicates, having fully cross-linked open framework structures made up of corner-sharing SiO4 and AlO4 tetrahedral groups. Zeolites belong to a class of minerals referred to generally as tectosilicates because their crystalline architecture is formed from silicon atoms in a tetrahedral, four-fold arrangement with oxygen atoms in a three dimensional lattice. The aluminosilicate structure is negatively charged and attracts the positive cations that reside within. Unlike most other tectosilicates, zeolites have large vacant spaces or cages in their structures that allow space for large cations such as sodium, potassium, barium and calcium and even relatively large molecules and cation groups such as water, ammonia, carbonate ions and nitrate ions. Zeolites are widely used as chemical sieves, odor absorbants, and dessicants.
Zeolites are known in the art for producing heat, almost instantaneously, upon hydration. Heat is generated as a result of adsorption of water within the structure of the zeolites. As long as zeolites stay dry, there will be no exothermic reaction. That is, zeolites undergo an exothermic reaction when transitioning from a dehydrated form to a hydrated form. A variety of anhydrous chemical formulations have been developed which contain such zeolites, so that the formulations may generate heat upon contact with water. Examples of such chemical formulations include a variety of personal care products such as lotions, cleansers, and other cosmetic formulations which generate heat when exposed to water, such as skin moisture, thereby producing a warming effect sensed by the skin. However, it is undesirable for this warming effect to occur too rapidly, become exhausted too quickly, or provide too much heat. With zeolites generating heat almost instantaneously upon contact with water, the warming effect of such cosmetic formulations is often intense and short-lived. A need therefore exists to delay the activation of the zeolite upon contact with water, such that heat is released continuously and gradually, and such that the warming effect lasts longer.